DESCRIPTION: A functional nervous system requires both the appropriate development of dendritic spines and their functional plasticity throughout life. Because dendritic spines are the primary sites of contact with presynaptic axons in excitatory neurons of hippocampus and cortex, their structure and function have been studied in great detail. Actin filaments (f-actin) play prominent roles in the formation, maintenance and plasticity of dendritic spine structure. However, the role of MTs in spine architecture has been studied little because spines are thought to be devoid of MTs. Prominent in dendrite shafts, MTs are assumed to function exclusively as stable railways for intracellular transport. However, MTs exhibit bouts of rapid polymerization and depolymerization, termed dynamic instability. Surprisingly, we discovered that MTs remain dynamic throughout neuronal development and are capable of rapidly extending into and out of dendritic filopodia and spines of cultured cortical and hippocampal neurons. Using total internal reflection fluorescence microscopy (TIRFM), we show that MT invasion of dendritic spines can be associated with rapid morphological changes of the spine head. These findings suggest that dynamic MTs may be playing an important role in spine structure and function. Indeed, many of the components that are either transported on MTs (lipids, proteins, RNA, organelles) or are associated with their growing tips would be capable of directly entering spines via MTs themselves. In this proposal we will test the hypothesis that dynamic MT entry into dendritic spines occurs in a regulated fashion and is required for spine development and plasticity. Specifically, we will: 1) Characterize the role of MT dynamics in spine morphology and maturation, 2) Determine how MTs affect synaptic activity and spine plasticity, and 3) Investigate MT-based targeting of synaptic components to dendritic spines. This work will provide fundamental insights into synaptogenesis and synaptic plasticity. Furthermore, because dendritic spines are the sites that are affected in numerous psychiatric and neurological diseases these studies hold promise for novel cytoskeletal-based therapies for synaptic dysfunction. PUBLIC HEALTH RELEVANCE: There are many developmental and adult onset neurological diseases, including mental retardation, autism, epilepsy, and Alzheimer's disease, that affect neuronal shape and therefore communication between neurons in the brain. This research will identify and characterize a novel intracellular mechanism that regulates directed movement of components to specific regions of the neuron and controls neuronal shape, thereby providing potential targets for therapies directed at ameliorating these diseases.